ABSTRACT: Bacterioplankton are the primary consumers of dissolved organic matter in the ocean, thus the quantification of bacterioplankton production (BP) is essential to our understanding of carbon cycling in the largest ecosystems on Earth. We compared BP, measured as the rate of 14C-leucine or 3H-leucine uptake at close to saturating concentration (20 nmol L−1), with ambient uptake measured from dilution bioassays. We hypothesized that saturation with leucine would lead to its respiration as a carbon source, thereby not truly representing ambient BP. Seawater was collected from the photic zone throughout the Atlantic Ocean. Respiration as a proportion of total consumption (uptake &plus; respiration) of close to ambient (0.4 nmol L−1) and close to saturating (20 nmol L−1) 14C-leucine concentrations were compared. Saturating 3H-leucine additions overestimated ambient leucine uptake at low rates (200% ± 100% ambient) and underestimated uptake at high rates (90% &plus; 20% ambient). The proportion of total leucine uptake that was respired was threefold higher for 20 nmol L−1 14C-leucine additions than 0.4 nmol L−1 14C-leucine additions (15% ± 8% and 5% ± 4%, respectively). Consequently, microbial efficiency of leucine assimilation—an indicator of bacterioplankton growth efficiency—was significantly higher and more stable at close to ambient 14C-leucine additions than at saturating concentrations (95% ± 4% and 85% ± 8%, respectively). Thus, saturation of oligotrophic open Atlantic Ocean bacterioplankton with leucine, or other molecules indicative of microbial metabolism, leads to the measurement of a response to a nutrient addition, rather than an ambient measurement.

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